This invention relates to improvements in retarding unwanted biological growth in food packaging particularly in retarding fungal or mould growth in packaged fruit especially table grapes.
Where table grapes are stored for-prolonged periods or where they are transported over long distances, the continued or repeated application of a fungicide to slow the growth of Botrytis mould is required.
Australian patent 565243 disclosed incorporating a fungicide in a packaging film of polyethylene and heating the film to release the fungicide. Australian patent 646013 discloses a wrapping film for use in retarding fungal decay of citrus fruits which incorporates a fungicide in a first resin layer of a high melt temperature [170-240xc2x0 C.] and then blending it with a second resin of lower melt temperature. This ensures that the fungicide is released slowly.
In Australia the current method of control is to package table grapes in a waxed corrugated cardboard box having an inner plastic liner bag. A sulphur dioxide releasing pad or sachet is placed inside the package. Such a system is disclosed in U.S. Pat. No 3,559,562. Although this has given satisfactory results there are a number of disadvantages. The rate of release of sulfur dioxide is not well controlled especially if there are temperature fluctuations during storage. High levels of Sulfur dioxide cause bleaching of the grapes with the consequence that their sale price is lowered. Further, high residue levels of sulfite in the grapes can cause health problems for a proportion of the population. Another difficulty with the present method is the localized release of sulfur dioxide within the package resulting in considerable variation of sulfur dioxide concentration.
It is an object of this invention to provide a means of releasing sulfur dioxide at a constant or zero order rate so that a minimally effective dosage rate can be achieved to reduce the disadvantages of using sulfur dioxide.
To this end the present invention provides a block, pad or film of a polymer blend which contains at least one polymer having a substantially different water transmission rate to at least one other polymer in the blend and there being dispersed through the polymer blend an antifungal agent activated by the presence of moisture.
This invention is predicated on the discovery that the release rate of water activated fungicides such as sodium metabisulfite can be controlled by controlling the proportions of polymers having high and low water transmission rates. This invention is predicated on the realisation that there are two polymer properties which affect the release rate of sulfur dioxide, the rate at which water particularly water vapour penetrates the polymer to contact the bisulfite and the rate of transmission of the sulfur dioxide through the polymer. This discovery means that slow release preparations can now be provided that have a lower release of sulfur dioxide without being below the concentration which inhibits fungal growth. The release rate can also be adjusted to suit the rate required by different fruit varieties. The property of the polymers most useful in determining the blend components is the water vapour transmission rate [WVTR].
Where the water activated fungicide is sodium metabisulfite the polymer blend preferably is processable at temperatures below 150xc2x0 C. which is the decomposition temperature of the sodium metabisulfite. Potassium metabisulfite as the fungicide allows processing temperatures below 190xc2x0 C. to be used but it has a slower release rate and a lower overall conversion per mole.
The polymer blend is preferably an olefine polymer with a polymer containing hydrophilic groups. The ratio of hydrophobic to hydrophilic polymer can be determined by simple trials. Preferably the polymer blend includes an ethylene/vinyl acetate copolymer (EVA) as the polymer with high release rate and a linear low density polyethylene (LLDPE) as the polymer with a low water release rate. A blend of these two polymers EVA/LLDPE within the range of 30:70 to 80:20 is usually effective. Preferred blends contain equal amounts of EVA and LLDPE or 70% EVA and 30% LLDPE.
The fungicide can be any active fungicide that allows the active agent to migrate through the polymer in a sustained manner. Water activated fungicides are preferred for the polymer blend of this invention. Sulfur dioxide generating agents are preferred and in particular bisulfite compounds the most preferred being sodium metabisulfite. The metabisulfite is in finely divided form, but there is no criticality to the particle size of the compound. The amount of the metabisulfite in the polymer blend must be sufficient to maintain a sustained release of sulfur dioxide over a period of up to 8 weeks. Usually a content of 10% to 30% by weight has been found to be adequate.
The metabisulfite can be mixed with one of the polymer components and then blended with the second polymer, This is not critical and the lower processing temperature is the key to selecting the steps in blending. With EVA/LDPE blends it is preferred to form a master batch of metabiswfite with EVA and then blend this with LDPE in the desired ratio to form the film.
The film of this invention can be used as a single film or as one layer in a laminate or coextruded multilayer film. The reason for doing this can be to improve the strength of the film, its printability or to modify the water transmission rates of the film. It is preferred to coextrude the EVA/LDPE blend containing the metabisulfite with LDPE. The LDPE layer provides a better printing surface and becomes the outside layer and decreases the penetration of water vapour from outside of the box.
In another aspect of this invention there is provided a package for transporting and storing fruit preferably table grapes which includes
1. a container;
2. optionally a water absorbent material lining said container;
3. optionally, a plastic liner between the container walls and said absorbent material;
4. disposed adjacent the fruit a film pad or block of a polymer blend containing a moisture activated antifungal agent wherein the polymer includes at least one polymer having a water transmission rate substantially different to that of at least one other polymer of said blend.
It has been found that the presence of a water absorbent material such as corrugated paper wadding appears to stabilize the release rate of sulfur dioxide presumably by reducing the humidity levels in the container. Some wadding materials may also act as a sink for sulfur dioxide.